Clutch-to-clutch upshifting in an automatic transmission involves an exchange of on-coming and off-going fluid operated friction elements (clutches), and is generally characterized as comprising three successive phases: a fill phase, a torque phase and an inertia phase. In the fill phase, the on-coming clutch is filled in preparation for torque transmission; in the torque phase, the torque exchange occurs without a corresponding speed change; and in the inertia phase, the speed change occurs.
The conventional method of performing open-loop clutch-to-clutch upshifting involves progressively increasing the fluid pressure supplied to the on-coming clutch while progressively releasing the fluid pressure applied to the off-going clutch. This results in a certain level of overlap during which both on-coming and off-going clutches are partially engaged. While the overlap is not particularly desirable from the standpoint of shift quality since the unnecessary torque capacity of the off-going clutch directly reduces the transmission output torque, it does serve to prevent engine flare if the torque capacity of the on-coming clutch is delayed due to underfilling of the on-coming clutch. Overfilling of the on-coming clutch causes a further reduction of the transmission output torque.
However, if the duration of the on-coming clutch fill period can be accurately determined in advance of the shift, the off-going clutch can be released just as the on-coming clutch develops torque capacity, permitting a smooth exchange of torque from the off-going clutch to the on-coming clutch with almost no overlap. This minimizes the transmission output torque disturbance in two ways: (1) it minimizes the output torque reduction during the torque phase and (2) it minimizes the duration of the torque phase so that the torque disturbance does not significantly excite the vehicle drivetrain components.
Nevertheless, there is a certain amount of variability due to manufacturing tolerances, temperature, wear and so on, that limits the ability of the control system to precisely estimate the on-coming clutch fill time under all conditions. This difficulty has been largely overcome through the use of adaptive control techniques which accurately learn the fill times (or fill volumes) of the various clutches over a number of shifts involving each clutch. A detailed example of such a system is set forth in the U.S. Patent to Downs et al. U.S. Pat. No. 4,707,789, issued Nov. 17, 1987, and assigned to General Motors Corporation.
In spite of the use of adaptive learning, it is difficult to compensate for certain types of variability (such as temperature-related variability), and convergence of the adaptively adjusted parameter (fill time) to its true value may only occur after a number of shifts to a particular speed ratio. This limits the ability of the control system to consistently achieve precise torque exchange between the off-going and on-coming clutches during the torque phase of a shift.